The present invention relates to methods for treating a subterranean formation in order to stabilize water-sensitive clays and migrating fines.
The recovery of fluids such as oil and gas from subterranean formations has been troublesome in formations that contain water-sensitive minerals, e.g., water-swellable clays, such as clays in the smectite group, and fines capable of migrating when disturbed, such as silica, iron minerals, and alkaline earth metal carbonates. The terms “clays” and “water-sensitive clays” are used herein interchangeably to generally indicate water-sensitive that, when contacted by aqueous fluids in disequilibrium with the minerals in the formation, tend to swell and/or migrate. Thus, the use of aqueous fluids such as injection fluids, drilling muds, and stimulation fluids in such formations may be problematic as the resulting swelling and migration tends to block passageways to the wellbore, thereby causing a loss in permeability of the formation.
This loss in permeability impairs the flow of fluid through the wellbore and, in some cases may even completely block the flow of fluids through portions of the formation. Loss in permeability often leads to a decrease in the rate of recovery of the well. Moreover, migrating clays can be produced with the formation fluids, thereby presenting abrasion and other problems with the production equipment.
In an effort to overcome these problems, various methods have been developed for treating subterranean formations to stabilize the clays against swelling and/or migrating. For example, it has been common practice to add salts to aqueous drilling fluids. The salts adsorb to the clay surfaces in an ion exchange process that can reduce the swelling and/or migration of the clays. Another method used to deter migration is to coat the region with a polymer and/or a consolidating resin in order to physically block the migration of the clays. The term “clay stabilizer solution” as used herein refers to any solution used to stabilize clay within a subterranean formation against clays swelling and/or migrating. The term “stabilizing components” as used herein refers to the stabilizing components of a clay stabilizer solution including, but not limited to, salts, polymers, resins, soluble organic stabilizing compounds, and combinations thereof.
When a clay stabilizer solution is exposed to water-sensitive clays, the stabilizing components are consumed by the clays through known mechanisms including adsorption, ion exchange, and chemical reaction. As the concentration of the stabilizing components decreases in the remaining solution, untreated water-sensitive clays are exposed to aqueous fluids which promotes swelling and migration. Current state-of-the-art implementation of clay stabilizer solutions call for injection of a single bolus of a relatively high concentration of clay stabilizer solution into the subterranean formation. Using such a method results in the depletion of clay stabilizing components most notably at the leading-edge of the clay stabilizer solution as the solution migrates through the subterranean formation.